1. Field of the Invention
This invention relates to a perpendicular magnetic recording medium mounted in various magnetic recording devices. More specifically, this invention relates to a perpendicular magnetic recording medium mounted in a hard disk drive (HDD) used as an external storage device of a computer, audio-video equipment, and similar.
2. Description of the Related Art
From 1997 on, there has been a rapid increase, at an annual pace of from 60 to 100%, in HDD recording densities. As a result of such remarkable growth, it is thought that the in-plane recording method that has been used in the past is approaching the limit of high-density recording. Given such circumstances, in recent years there has been much interest in perpendicular recording methods enabling higher recording densities, which has been the subject of much active research and development. And from 2005, at long last, HDDs adopting perpendicular recording methods in some models have been commercialized.
A perpendicular magnetic recording medium principally includes a magnetic recording layer of hard magnetic material; an underlayer to orient the magnetic recording layer in a target direction; a protective film to protect the surface of the magnetic recording layer; and a backing layer of soft magnetic material, which serves to concentrate the magnetic flux generated by a magnetic head used in recording onto the magnetic recording layer.
In order to improve the basic characteristics of the medium, the signal output-to-noise ratio (S/N) must be improved. That is, the signal output from the medium must be increased, and noise must be reduced. One cause of a decrease in signal output and an increase in noise is an increase in orientation dispersion (crystal orientation irregularity) in the magnetic recording layer. In the perpendicular magnetic recording medium, the magnetization easy axis in the magnetic recording layer must be oriented perpendicular to the medium surface. If there is large dispersion in the orientation of this easy axis of magnetization, then the consequent reduction in the perpendicular-direction flux causes a decline in signal output. According to the results of studies by the inventors, in a medium with large orientation dispersion, the magnetic discreteness of crystal grains declines and magnetic cluster sizes increase, and noise is increased (see Shunji Takenoiri et al., “Development and problems of CoPtCr—SiO2 perpendicular media”, 135th Topical Symposium Magn. Soc. Jpn. (2004)).
In addition, there have been proposals in the prior art in order to improve the electromagnetic transducing characteristics by improving magnetic characteristics and reducing noise arising in the soft magnetic backing layer. One proposes to use a two-layer underlayer of an alloy of Fe, Cr or Co and Ru as an underlayer or intermediate layer positioned between the magnetic recording layer and the soft magnetic backing layer (see Japanese Patent Laid-open No. 2002-100030, corresponding U.S. patent application 2002/0058160A1, corresponding Singapore patent application 91345A1). Another proposes to use a CoFe alloy soft magnetic backing layer and Ru underlayer (see Japanese Patent Laid-open No. 2002-298323).
It has further been proposed to use a soft magnetic Permalloy material as an underlayer, and to use Ru or a Ru-base alloy as a comparatively thick nonmagnetic intermediate layer, so that orientation dispersion in the magnetic recording layer can be reduced, the initial growth layer can be decreased, crystal grain diameters can be reduced, and similar (see Japanese Patent Laid-open Nos. 2002-358617 and 2003-123239). Further, it has been proposed, when using a soft magnetic Permalloy material underlayer and an Ru or Ru-base alloy material intermediate layer, to insert a soft magnetic Co layer or a soft magnetic Co-base alloy layer between the underlayer and intermediate layer. In this way, the film thickness of the intermediate layer can be reduced, and at the same time the coercive force and squareness ratio of the magnetic recording layer can be increased, and the S/N at recording densities used in the prior art can be improved (see Japanese Patent Laid-open No. 2004-288348).
However, there remains a need for the perpendicular magnetic recording medium which affords high signal output and low noise even during high-density recording to attain a higher S/N, in keeping with trends toward still higher recording densities. In order to realize a higher S/N through increased signal output and reduced noise of the perpendicular magnetic recording medium, it is necessary that the orientation dispersion of the magnetic recording layer be reduced as much as possible.
In addition to the above points, in order to lower noise in the magnetic recording medium, it is necessary to decrease the crystal grain diameters in the magnetic recording layer. If the crystal grain diameters in the magnetic recording layer are large, then bit transition regions become irregular, and transition noise increases. Hence in order to reduce the transition noise it is necessary to reduce the crystal grain diameters, and to make the bit transition regions linear.
From the standpoint of raising the recording density of the perpendicular magnetic recording medium as well, it is necessary to reduce noise in bit transition regions. To this end, securing sharp recording magnetic fields and making transitions as linear as possible are effective. In order to obtain a sharp recording magnetic field, the distance between the soft magnetic backing layer and the magnetic head must be made as small as possible. Further, because the recording magnetic field of the magnetic head declines as the recording density rises, the distance between the soft magnetic backing layer and the magnetic head must also be reduced in order to secure an adequate recording magnetic field.
In general, a nonmagnetic underlayer or an intermediate layer is provided between the magnetic recording layer and the soft magnetic backing layer. At present, however, the nonmagnetic underlayer or intermediate layer is thick, at a film thickness of approximately 20 to 30 nm, and is a factor increasing the distance between the soft magnetic backing layer and the magnetic head. In actuality, in currently proposed configurations as described above, the nonmagnetic underlayer or intermediate layer is very thick (for example, 35 nm or larger in the configurations of Japanese Patent Laid-open No. 2002-100030, corresponding U.S. patent application 2002/0058160A1, corresponding Singapore patent application 91345A1, and Japanese Patent Laid-open No. 2002-298323). Such a thick underlayer or intermediate layer is inadequate for shortening the distance between the magnetic head and the soft magnetic backing layer and obtaining a high S/N in high-density recording.
Further, the underlayer or intermediate layer also functions to control the crystallinity, orientation, and crystal grain diameters in the magnetic recording layer formed thereupon, and affects the characteristics of the magnetic recording layer. In particular, when the magnetic recording layer is formed by epitaxial growth on the underlayer or intermediate layer, the crystal grain diameters of the magnetic recording layer material conform to the crystal grain diameters of the material in the underlayer or intermediate layer. Hence in order to reduce the crystal grain diameters in the magnetic recording layer material, it is effective to reduce the crystal grain diameters in the underlayer or intermediate layer. However, when the film thickness of the underlayer or intermediate layer is reduced, not only a lessening of the crystal orientation of the magnetic recording layer material, but also impediments to the magnetic isolation between magnetic crystal grains occur. Accordingly, the magnetic characteristics of the magnetic recording layer decline. In light of these facts, rather than simply reducing the film thickness of the underlayer or intermediate layer, it is necessary to maintain or improve the magnetic characteristics of the magnetic recording layer while simultaneously reducing the film thickness.
Hence an object of this invention is to provide a perpendicular magnetic recording medium in which, simultaneously with reduction of the orientation dispersion of and shrinking of crystal grain diameters in the magnetic recording layer, the film thickness of the underlayer or intermediate layer can be reduced, and by this means performance improvements such as reduced noise, an improved S/N, and improved write-ability are made possible.